Lung endothelial barrier integrity at the level of adherens junctions (AJs) is required for lung fluid homeostasis. A crucial mechanism contributing to the loss of endothelial barrier integrity in conditions such as pulmonary edema is ?stress failure? of pulmonary capillaries in response to high pressure. While it is known that AJs, comprised of VE-cadherin and associated catenin proteins, restrict endothelial permeability, little is known about how mechanical forces, specifically vessel wall tension, control endothelial permeability and pulmonary edema. Our Supporting Data describe the potentially important role of hydrostatic pressure in microvessels in activating the mechanosensor Piezo1 in endothelial cells (ECs) and in increasing endothelial barrier permeability. We observed that activation of Piezo1 induced intracellular Ca2+ signaling, which in turn, caused phosphorylation of VE-cadherin and increased microvascular permeability. These findings have for the first time linked increased tension to which ECs are exposed to the activation of Piezo1 and disassembly of AJs, leading to the fundamental question ?how does tension sensed at the plasma membrane of ECs by Piezo1 activate VE- cadherin phosphorylation and thereby disrupt AJs?? In Aim 1, we will determine the role of microvessel pressure in activating the mechanosensor channel Piezo1 in lung ECs and Piezo1?s role in regulating endothelial permeability and lung fluid balance. We will determine whether Src dependent phosphorylation of Piezo1 is required for Piezo1 activated Ca2+ signaling in ECs and whether this thereby mediates increased endothelial permeability. In Aim 2, we will determine the role of Piezo1 signaling in mediating disassembly of AJs through phosphorylation of VE-cadherin, and in increasing endothelial permeability. Here, we will identify the signaling pathway downstream of Piezo1 activation that induces phosphorylation of VE-cadherin and VE-cadherin endocytosis and thus disassemble the AJs. In Aim 3, we will determine the role of Piezo1 in mediating lung vascular hyper-permeability (?stress failure?) and edema associated with left heart failure (LHF). These studies will address the pathophysiological relevance of Piezo1 in the mechanism of pulmonary edema resulting from LHF-induced increases in lung microvessel pressure. The above studies will be essential for understanding the role of Piezo1 in increasing lung microvessel permeability, with the goal of identifying new therapeutic targets for high pressure-induced pulmonary edema.